The field of the invention relates generally to a method for preparing very-high or ultra-high molecular weight polyethylene. More particularly, the present invention relates to a method of preparing very-high or ultra-high molecular weight polyethylene using a supported catalyst comprising a support, an activator and a metal-ligand complex, as well as the catalyst itself. The present invention additionally relates to a method of using a supported catalyst comprising a support, an activator and co-supported metal-ligand complexes to obtain very-high or ultra-high molecular weight polyethylene with a bi-modal molecular weight distribution.
Ancillary (or spectator) ligand-metal coordination complexes (e.g., organometallic complexes) and compositions are useful as catalysts, additives, stoichiometric reagents, solid-state precursors, therapeutic reagents and drugs. Ancillary ligand-metal coordination complexes of this type can be prepared by combining an ancillary ligand with a suitable metal compound or metal precursor in a suitable solvent at a suitable temperature. The ancillary ligand contains functional groups that bind to the metal center(s), remain associated with the metal center(s), and therefore provide an opportunity to modify the steric, electronic and chemical properties of the active metal center(s) of the complex.
Certain known ancillary ligand-metal complexes and compositions are catalysts for reactions such as oxidation, reduction, hydrogenation, hydrosilylation, hydrocyanation, hydroformylation, polymerization, carbonylation, isomerization, metathesis, carbon-hydrogen activation, carbon-halogen activation, cross-coupling, Friedel-Crafts acylation and alkylation, hydration, dimerization, trimerization, oligomerization, Diels-Alder reactions and other transformations.
One example of the use of these types of ancillary ligand-metal complexes and compositions is in the field of polymerization catalysis. In connection with single site catalysis, the ancillary ligand typically offers opportunities to modify the electronic and/or steric environment surrounding an active metal center. This allows the ancillary ligand to assist in the creation of possibly different polymers. Group 4 metallocene based single site catalysts are generally known for polymerization reactions. See, generally, “Chemistry of Cationic Dicyclopentadienyl Group 4 Metal-Alkyl Complexes”, Jordan, Adv. Organometallic Chem., 1991, Vol. 32, pp. 325-153 and the references therein, all of which is incorporated herein by reference. One application for metallocene catalysts is in the production of polyolefins, such as in the production of polyethylene.
A type of polyethylene of particular value is ultra-high molecular weight polyethylene (“UHMWPE”). Ultra High Molecular Weight Polyethylene is a valuable engineering plastic, with a unique combination of abrasion resistance, surface lubricity, chemical resistance, and impact strength, and very high tensile strength as a fiber. See, for example, Stein, H. L., Ultra High Molecular Weight Polyethylene (UHMWPE), pp. 167-171, in ENGINEERED MATERIALS HANDBOOK, Volume 2: Engineering Plastics, ASM International, 1998. Industrial uses include, for example, liners for bulk material handling, nautical rope, truck bed linings and metal shaft bushings. UHMWPE is the product of a cheap monomer (ethylene) and a relatively simple process (typical slurry HDPE processes), using fairly conventional Ziegler catalysts. See, for example, U.S. Pat. No. 5,587,440 and EP 0575840 B1.
Ultra-high molecular weight polyethylene may typically be characterized by a molecular weight of at least about 3×106 g/mol, with molecular weights from about 3×106 g/mol to about 10×106 g/mol being typical. In contrast, very-high molecular weight polyethylene may typically be characterized by a molecular weight from about 1×106 g/mol to less than about 3×106 g/mol and high molecular weight polyethylene may typically be characterized by a molecular weight of greater than about 3×105 g/mol to less than about 1×106 g/mol. Conventional UHMWPE resin does not exhibit a measurable melt index and cannot be processed using conventional polyolefin melt processing techniques such as, for example, injection molding, blow molding, rotomolding or film blowing or casting. Rather, UHMWPE is conventionally processed by compression molding or ram extrusion. Compression molding and ram extrusion are relatively slow processing techniques and require products to be machined from the resulting sheets or rods. The main limitation to wider use of UHMWPE is the difficulty of processability.
Broad or bimodal molecular weight distribution polymer compositions are compositions that typically include one or more high molecular weight polymers and one or more low molecular weight polymers. In bimodal molecular weight distribution polymer compositions, the weight fraction of the high molecular weight polymer may range from, for example, 0.10 to 0.90. The relative amount of high molecular weight polymer in the polymer composition can influence the rheological properties of the composition. One such measurable rheological property of bimodal polymer compositions is its melt flow rate (e.g. I21, measured at 190° C., with a 21.6 kg load according to ASTM D-1238). By increasing the weight fraction of low molecular weight polymers in the polymer composition, the polymer composition may generally exhibit improved flow characteristics.
Conventional techniques to improve the processibility of UHMWPE involve the melt blending of a lower molecular weight polymer with UHMWPE compositions, or involve use of two reactors in series. Such techniques have generally proven to be insufficient due to difficulty in uniformly dispersing the lower molecular weight polymer into the composition. Such poorly blended compositions are characterized by a decrease in impact strength and wear resistance compared to unblended UHMWPE.
While the melt processability to the UHMWPE can be greatly improved by blending with lower molecular weight polymers, this comes at the price of reduction in the key desirable properties of UHMWPE. One problem is the difficulty of achieving a homogeneous blended product. The extremely low melt viscosity of the UHMWPE makes it very difficult to fully dissolve & disperse the UHMWPE particles, resulting in a “pumpable slurry” in the worst cases. This results in a marked decrease in impact strength and wear resistance compared to unblended UHMWPE. See, for example, U.S. Pat. Nos. 4,110,391, 4,281,070, 4,786,687, 4,923,935, 5,079,287, 5,393,473, 5,422,061, 5,422,061, 5,658,992, 6,521,709, 6,790,923, and WO 02/046297.
Melt-processable blends of UHMWPE and HDPE (high density polyethylene) have also been prepared using 2-stage reactor technology. Typically, ethylene is polymerized in the absence of hydrogen to produce UHMWPE in the first stage, then in the presence of hydrogen to produce lower molecular weight HDPE in the second stage. Resulting granular products are intra-granular blends. See for example U.S. Pat. No. 4,786,687, EP 0274536 B2, both employing conventional Ziegler catalysts.
It has been demonstrated that bimodal polyethylenes may be prepared by simultaneous polymerization of ethylene (and optionally α-olefin comonomer(s)) to produce a lower molecular weight polyethylene component and a high molecular weight polyethylene component by use of co-supported “bimetallic” catalysts in a single reactor (see, for example, U.S. Pat. Nos. 5,032,562, 5,539,076, 5,614,456, 6,051,525, WO 02/090393, WO 02/44222, and WO 03/048213, the entire contents of which are incorporated herein by reference for all relevant and consistent purposes). The resulting compositions possess a high degree of dispersion due to the intra-granular blending that occurs during the simultaneous polymerization. Compared to series-reactor products, improved intra-granular blending is possible by growing both components simultaneously. While these in-reactor blends produced by use of co-supported catalysts in a single reactor have been demonstrated for polyethylenes with regular and high molecular weights, catalyst systems capable of producing bimodal ultra-high molecular weight polyethylene have not been effectively demonstrated.
UHMWPE fibers are typically produced using a gel spinning process, typically using a 2-step process that produces fibers with highly oriented UHMWPE chains, resulting in superb tensile strength. See, for example, U.S. Pat. Nos. 4,137,394, 4,356,138, 4,413,110, and 7,147,807. UHMWPE compositions with narrow molecular weight distributions may offer improved properties for fiber applications.
In view of the foregoing, a need continues to exist for catalyst compositions that may be used to prepare ultra-high molecular weight polyolefins, and in particular UHMWPE compositions, with desirable molecular weight distribution (MWD), either narrow MWD (e.g. for fiber applications), or bimodal MWD (e.g. for improved melt flow properties). Additionally, a need exists for catalyst compositions that may be used to produce UHMWPE compositions with a bimodal molecular weight distribution, thus avoiding the need for blending and problems associated therewith. A further need exists for methods of producing UHMWPE that provide for the production of such polymers that have a specific target molecular weight and molecular weight distribution.